The Gen2 RFID protocol includes an amplitude modulation (AM) mode, referred to as phase reversal amplitude shift keying (PR-ASK) in the protocol specification, which can achieve good spectral occupancy. However, the amplitude of the radio frequency (RF) signal passes through zero. This gives 100% amplitude modulation depth and produces phase discontinuities with 180 degree phase jumps. Many RFID readers are designed using RF power amplifiers (RFPAs) with high power efficiency, such as class-AB or class-C. These high efficiency power amplifiers tend to have reduced linearity as compared to class-A amplifiers. These high efficiency power amplifiers work better when the amplitude modulation of the transmission signal is reduced. While ideal PR-ASK modulation can achieve good spectral occupancy, the 100% AM depth and the phase discontinuities can create significant distortion for power efficient RFPAs commonly used in RFID readers. This distortion causes spectral regrowth which can significantly degrade the spectral occupancy.
Some RFID reader designs include nonlinear predistortion on the baseband transmission signal to improve overall transmitter linearity and mitigate the spectral regrowth problem. This nonlinear predistortion is calculated using input-output characterization of the RFPA. The input-output characterization can be very difficult to achieve accurately when the transmission signal modulation depth is at or near 100%, such as with PR-ASK. If digital predistortion is used, the amplitude and phase distortion of the RF power amplifier is difficult to measure when the RF signal trajectory goes through the origin. Thus, ideal PR-ASK has good spectral occupancy but is difficult for many power efficient RFPAs to reproduce linearly and furthermore is difficult to implement RFPA predistortion for due to its deep amplitude modulation depth.